the obvious argument for "open source first" is economic
For the most part, bringing up a given device in Windoze is going to be a matter of updating the driver(s). That gets the hardware vendor a chance at the extremely overcrowded Windows market said vendor already had access to as of the last Windows-only model.
Doing the work needed to add "Linux compatible" to the device web page gives one access to the overwhelmingly Linux server market and of course, the small but growing Linux desktop market. Even computers not intended for server usage will work at the low end of the webserver spectrum, and a vendor who doesn't care to support Linux will simply have professional customers go somewhere else.
Everyone who puts "Linux compatible" and [device type] into google will have a list with the vendors that sells Linux compatible devices on them.
And for every distro that's listed on a given device page, the chances that people will find that device increase.
Vendors spend lots of money on search engine optimization.
The best way to increase one's page hit rate and sales if one is selling commodity computer hardware is to have keywords on a page like in this case, Linux , Debian, Ubuntu, SUSE, Red Hat, and for that matter, OpenBSD and Solaris. It should be obvious that the more keywords that describe product features, the better for sales. It only takes a few sales to pay for the developer time required to make any given product Linux-compatible, after that, Linux sales are simply more profit.
NOT doing this is leaving money on the table, doing this on purpose or due to ignorance is an unsound business practice.